1. Field of the Invention
This invention relates to improvements in gravity filters which employ multiple filter cells having mono-media or dual-media filter beds which are adapted to be individually backwashed by effluent from companion filter cells. In particular, the invention comprises a novel, centrally-located, filtered water effluent control chamber which communicates with each filter cell and contains an adjustable circular weir or, alternatively, a pumped backwashing system, which is used to control the backwashing process.
2. Description of the Related Art
It is known in the prior art that a plurality of filters can be connected in parallel with common influent and effluent passages and with provisions for backwashing one or more filters with the effluent of companion filters, while the other units continue their normal filtering operations. Examples of these type of filters (referred to hereinafter as self-backwashing filters) will be found in U.S. Pat. Nos. 3,134,735 and 3,282,432, both issued to Greenleaf. Self-backwashing filters normally require substantially deeper filter boxes than conventional filters that use backwash pumps in order to provide sufficient head for fluidized-bed backwashing. The need for deep filter boxes (typically greater than 20 feet) has precluded the use of such filters in package water treatment systems, which are limited to a maximum tank depth of 12 feet to comply with shipping requirements.
It is also known that it is difficult to adjust the rate of backwashing in self-backwashing filters to compensate for changes in the temperature of the backwash water, provide higher backwash rates if the filter beds are difficult to clean or lower rates to prevent loss of filter media. The backwash rate is normally adjusted by changing the height of a common filtered water effluent weir by adding or removing stop logs. This is a tedious procedure and seldom done in practice. It also does not allow backwash rates to be adjusted during a backwash cycle to optimize the backwash process.
It is further known that a system of interconnecting filter underdrains is required for backwashing one or more filters with the effluent of other filters and that for this reason individual filter cells cannot be isolated and removed from service for inspection or repairs unless large isolation valves are installed between adjacent filter underdrains. Such valves are also generally required when air scour systems are used for cleaning filter beds in order to prevent backwash water from entering the filter cell while the bed is being air scoured. Isolation valves must be large enough to minimize pressure drops across valve openings during backwashing. They also add considerably to the operational complexity and cost of the filter system.
Thus, although self-backwashing filters are simple and effective filtering devices which do not require as much mechanical equipment (e.g., pumps, motors, rate-of-flow controllers) as conventional gravity filters, they are characterized by several shortcomings including the need for deep filter boxes, a relatively inflexible method for adjusting backwash rates, and the need for additional valves for isolating filter cells during repairs or when air scour systems are used. For these reasons, they seldom have been used in package water treatment plants or where site constraints preclude the use of deep filter boxes.